This is why I commented that Plasma is more print-disabled friendly than Gnome as that is the source of the change not Zorin. That said I'd like @Storm who is running latest Plasma whether solid backgrounds are still available. I think because of this it was inappropriate of me to state on my website that it has accessibility out of the box. I think I will have to drop Zorin in favour of Plasma distros on my site, the ultimate being Devuan that can be installed with a Braille Device, no sighted person's help required.
Unfortunately in gnome-tweaks, You can't customize the Login Screen. You can only set up Background Pictures for the Desktop and the Lock Screen.
I thought that was what the OP was asking for. 
I've never seen any other way to customize the lock screen.
You have to seperate the Lock Screen and the Login Screen. They might look the same, but these are different Things. And zab wrote:
Here is a thread how to change the loginscreen. The easiest way is to switch from gdm to lightdm.
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Or like at the End of Your linked Thread shown, choose Version 2.0 (not 2.1 or any other) AppImage of GDM Settings (old Name: Login Manager Settings):